Additionally, climatologists look at natural changes in air and ocean currents like El Niño and La Niña, which are phases in a fluctuating cycle of air and ocean temperature over the Pacific Ocean. The oscillation between the warm El Niño and the cold La Niña phases affect climates around the world. These ocean current patterns result in changes in the normal difference between atmospheric and ocean temperatures. Scientists also consider the effects that solar activity and variations in solar energy have on climate over time. Some natural events can contribute to global warming, such as volcanic eruptions, which release large amounts of ash and other substances into the atmosphere.